Ink jet printing is accomplished by ejecting ink from jets toward paper or other medium upon which printing is to be placed. In various printing processes ink is driven from the jets toward the medium in different ways. Among these are electrostatic printing which is performed by driving ink from a jet toward a medium with an electrostatic field.
Another ink jet printing procedure employs a piezoelectric element located within the ink jet. Electrically caused distortions of the piezoelectric element cause it to operate in a pump-like fashion to drive ink through the jet and toward a printing medium. Still another ink jet printing procedure drives ink from the jet and toward the printing medium by forming an expanding vapor-phase bubble near the jet. The vaporizable material that forms the bubble is called the driver. These various printing methods are described in Vaught et al U.S. Pat. No. 4,490,728, Output Hard Copy Devices, edited by Durbeck and Sherr, Academic Press 1988, see particularly Chapter 13 entitled INK JET PRINTING, and EPC Application EP 0036790.
Some ink jet inks are liquids at room temperature. Liquid inks do not respond the same to different printing mediums. Typically, liquid ink on office papers will produce a feathered appearance because it penetrates and spreads into the paper following fiber lines. Liquid inks without curable additives may not be useful on non-porous surfaces such as metal, glass or plastic because, being liquid phase at room temperature, they smear. Liquid inks that are designed for minimum feathering require time to set which may limit the rate that printed pages are stacked and liquid inks are very sensitive to temperature changes which influence the ink viscosity and interfacial tension which in turn influences how the ink interacts with the medium.
Ink jet inks that experience thermally induced phase change have also been used. These inks are called solid hot melt or wax inks and are used in printing processes known as coldsetting processes. Cold-setting processes use heat to melt the solid ink within the jet assembly and then the liquid is driven through the jet mechanically, for example by using a piezoelectric element as described above. Conventional cold-setting ink jet inks are not useful in a thermal ink-jet printing process because they consist of low vapor pressure constituents which are not good bubble formers. The bubble former, called a driver, must have critical properties for any given jet configuration in order to initiate jetting by a thermal ink jet device. The high boiling waxy materials from which solid inks are made normally have low critical pressures and poor thermal diffusivity so that they cannot achieve to the high superheat temperatures required for sudden nucleation and explosive bubble formation.
Ink jet inks comprise a vehicle and a coloring element that is called a colorant. The vehicle is the portion of the ink that carries the colorant and it is usually an organic material such as an oil or a wax. The vehicle holds the ink to the medium while the colorant entrained in the vehicle produce the visible image. The term colorant as used in the ink jet ink art is a general term for a material that makes ink intensely colored and therefore visible when it is on the medium. The term colorant includes dyes and pigments.